This application is directed to infrared detectors, and more particularly to compensating for non-uniformities among detector elements of an infrared detector array.
Infrared detectors provide thermal images for temperature measurement and heat detection. They are used for various applications, such as for military, industrial, and medical applications. In its simplest form, an infrared detector is a device, such as a photosensitive diode, that generates an electric current when exposed to infrared radiation. This current is dependent on the intensity and wavelength of the radiation and can be used in many different ways to produce an infrared picture.
Infrared detectors may be configured as a single element (detector), a small array of elements, a long linear array, or a full two-dimensional array. When the detector is a full two-dimensional array, the entire image is recorded at once, and the array is referred to as a “staring” array. However, with smaller arrays, the image is scanned over the array. The small array requires a serial scan to sweep the image in two-dimensions, whereas the linear array requires a “pushboom” scan to sweep the image across the array in one dimension.
The current produced by an infrared detector is amplified and processed to provide a more useful detector output. The processing reduces interference due to external and internal causes, such as electrical noise.
The ideal response of an infrared detector array is that each detector element exhibit the same linear voltage response for given temperature changes in the irradiation of the array. However, one type of interference with a good detector signal is electrical noise due to detector non-uniformity among detector elements. The uniformity differences have both spatially and temporally dependent causes.
A number of methods have been tried for compensating non-uniformity of infrared detector arrays. Generally, all involve some sort of data processing. Some methods use a uniform calibration source, typically using a chopper and controlled temperature. Other methods are scene-based, which means that they use an image scene (with uniform temperature) comprised of one or more objects or patterns for calibration. Such image scene may provide better calibration than other methods for the detector array's non-linear response to temperature variation. The scene-based methods may be further categorized into mechanical and non-mechanical methods.
Mechanical methods include methods that use choppers (or defocused lens, which may be configured to blur the image scene, and thus providing a locally constant temperature of the scene), dither mirrors, or other devices to blur the scene or otherwise induce motion. The “dithered scan” method of non-uniformity compensation is a scene-based mechanical method. The detector array views a scene through suitable optics. During a given time frame, the incident flux is sensed by each detector element. At the end of the time frame, the array data is delivered for processing and the array is displaced (“dithered”) a fixed distance, typically a distance equal to the width or height of one detector element, in either the horizontal or vertical direction. Conventional dither scan methods assume the scene flux to be stable throughout the dither cycle. Thus, during the next time frame, each detector element is exposed to the flux seen by one of its neighbors during the prior time frame. These detector pairs can be “linked” analytically, such as by averaging their outputs. By a suitable choice of a dither pattern, each detector can be linked with one or more of its neighbors, to adjust gain and offset differences. Dithered scan methods are described in U.S. Pat. No. 5,925,880, to C. J. Young, et al., entitled “Non-Uniformity Compensation for Infrared Detector Arrays”.
Scene-based non-mechanical methods may be based not only on scene information, but also information related to continuous scene or platform motion. These methods have included temporal high pass filtering, neural networks, and constant statistics. What is needed is a system and method for compensating non-uniformities among detector array elements without the use of dither mirrors (or other mechanical methods), and without the requirement of continuous motion in the scene exposed to the detector array.